Some interviews require you to share your screen for the duration. Where they do, you are asked before the interview begins.
Sharing correctly
You will usually be asked for your entire screen, not a window or a tab. A window share hides everything outside it, which is the thing the requirement exists to see.
The browser's own dialog decides this. Choose the Entire Screen option and confirm.
A blank or black share
The commonest failure, and it is an operating-system permission rather than anything you did.
macOS — System Settings → Privacy & Security → Screen Recording, enable your browser, then quit the browser fully and reopen. macOS will not apply this without a restart of the application, and until you do the browser hands over a picture that is entirely black. The interview detects that and asks again rather than storing a black rectangle as if it were evidence.
Windows — usually works without a system permission. If the picture is black, update the graphics driver or try a different browser.
Stopping accidentally
Browsers show a floating "Stop sharing" control, and it is easy to click by accident. If you stop, the interview tells you and asks you to share again — it does not fail you.
Inside Safe Exam Browser
SEB manages the screen itself. Where a stage runs in SEB you will not be asked to share, because the lockdown already covers what the share would have shown.
Privacy
Close anything you do not want seen before you start. A shared screen is exactly what it sounds like, and the honest way to protect something private is not to have it on the screen you are sharing.